INSTEAD OF trigger for views in PostgreSQL - Mini Project: Build & Apply

Using INSTEAD OF Trigger for Views in PostgreSQL

📖 Scenario: You are managing a bookstore database. You want to allow users to update book prices through a view that combines book details and prices. Since views are read-only by default, you will create an INSTEAD OF trigger to handle updates on the view.

🎯 Goal: Create a view for book details and prices, then write an INSTEAD OF trigger on the view to update the underlying table when the view is updated.

📋 What You'll Learn

Create a table called books with columns id, title, and price.

Insert three specific books with given ids, titles, and prices.

Create a view called book_prices selecting id, title, and price from books.

Create an INSTEAD OF trigger function called update_book_price that updates the price in books when the view is updated.

Attach the INSTEAD OF trigger to the book_prices view.

💡 Why This Matters

🌍 Real World

Views with INSTEAD OF triggers let you create user-friendly interfaces to complex data, allowing updates through views that normally are read-only.

💼 Career

Database developers and administrators often use INSTEAD OF triggers to maintain data integrity and provide flexible data access in enterprise applications.

1

Create the books table and insert data

Create a table called books with columns id as integer primary key, title as text, and price as numeric(6,2). Insert these rows exactly: (1, 'The Great Gatsby', 10.99), (2, '1984', 8.99), (3, 'To Kill a Mockingbird', 12.50).

PostgreSQL

-- Create the books table and insert the three books
-- Your code here

Hint

Use CREATE TABLE with the specified columns and types. Then use INSERT INTO books with the exact values.

2

Create the book_prices view

Create a view called book_prices that selects id, title, and price from the books table.

PostgreSQL

CREATE TABLE books (
    id INTEGER PRIMARY KEY,
    title TEXT,
    price NUMERIC(6,2)
);

INSERT INTO books (id, title, price) VALUES
(1, 'The Great Gatsby', 10.99),
(2, '1984', 8.99),
(3, 'To Kill a Mockingbird', 12.50);

-- Create the book_prices view selecting id, title, and price from books
-- Your code here

Hint

Use CREATE VIEW book_prices AS SELECT id, title, price FROM books;

3

Create the INSTEAD OF trigger function

Create a trigger function called update_book_price in PL/pgSQL that updates the price column in the books table when the view is updated. The function should update the row where id = NEW.id and return NEW.

PostgreSQL

CREATE TABLE books (
    id INTEGER PRIMARY KEY,
    title TEXT,
    price NUMERIC(6,2)
);

INSERT INTO books (id, title, price) VALUES
(1, 'The Great Gatsby', 10.99),
(2, '1984', 8.99),
(3, 'To Kill a Mockingbird', 12.50);

CREATE VIEW book_prices AS
SELECT id, title, price FROM books;

-- Create the trigger function update_book_price
-- Your code here

Hint

Use CREATE FUNCTION with RETURNS trigger. Inside, update books setting price = NEW.price where id = NEW.id, then RETURN NEW.

4

Attach the INSTEAD OF trigger to the view

Create an INSTEAD OF UPDATE trigger on the book_prices view that calls the update_book_price function.

PostgreSQL

CREATE TABLE books (
    id INTEGER PRIMARY KEY,
    title TEXT,
    price NUMERIC(6,2)
);

INSERT INTO books (id, title, price) VALUES
(1, 'The Great Gatsby', 10.99),
(2, '1984', 8.99),
(3, 'To Kill a Mockingbird', 12.50);

CREATE VIEW book_prices AS
SELECT id, title, price FROM books;

CREATE FUNCTION update_book_price() RETURNS trigger AS $$
BEGIN
    UPDATE books SET price = NEW.price WHERE id = NEW.id;
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

-- Create the INSTEAD OF UPDATE trigger on book_prices view
-- Your code here

Hint

Use CREATE TRIGGER with INSTEAD OF UPDATE ON book_prices and call update_book_price().

Practice

1. What is the main purpose of an INSTEAD OF trigger on a view in PostgreSQL?

easy

A. To speed up queries on the view

B. To run custom code instead of the default insert, update, or delete on the view

C. To automatically create indexes on the view

D. To prevent any changes to the view data

Solution

1. Step 1: Understand what views normally do

   Views in PostgreSQL are virtual tables that do not store data themselves and usually cannot be directly updated.

2. Step 2: Role of INSTEAD OF triggers

   INSTEAD OF triggers let you define custom actions that run instead of the usual insert, update, or delete on the view, making it updatable.

3. Final Answer:

   To run custom code instead of the default insert, update, or delete on the view -> Option B

4. Quick Check:

   INSTEAD OF triggers = custom code on views [OK]

Hint: INSTEAD OF triggers replace default actions on views [OK]

Common Mistakes:

• Thinking INSTEAD OF triggers speed up queries
• Confusing INSTEAD OF triggers with indexes
• Believing INSTEAD OF triggers block all changes

2. Which of the following is the correct syntax to create an INSTEAD OF trigger on a view named my_view for INSERT operations?

easy

A. CREATE TRIGGER trg_instead_of_insert ON my_view INSTEAD OF INSERT FOR EACH ROW EXECUTE FUNCTION trg_func();

B. CREATE TRIGGER trg_instead_of_insert INSTEAD OF INSERT ON my_view EXECUTE FUNCTION trg_func();

C. CREATE TRIGGER trg_instead_of_insert ON my_view INSTEAD OF INSERT EXECUTE FUNCTION trg_func();

D. CREATE TRIGGER trg_instead_of_insert ON my_view BEFORE INSERT EXECUTE FUNCTION trg_func();

Solution

1. Step 1: Recall correct trigger syntax in PostgreSQL

   The syntax requires specifying the trigger name, the event timing (INSTEAD OF), the event type (INSERT), the target (ON my_view), and the function to execute.

2. Step 2: Identify the full correct syntax

   CREATE TRIGGER trg_instead_of_insert ON my_view INSTEAD OF INSERT FOR EACH ROW EXECUTE FUNCTION trg_func(); correctly includes "ON my_view INSTEAD OF INSERT FOR EACH ROW EXECUTE FUNCTION trg_func();" which is the proper syntax for INSTEAD OF triggers on views.

3. Final Answer:

   CREATE TRIGGER trg_instead_of_insert ON my_view INSTEAD OF INSERT FOR EACH ROW EXECUTE FUNCTION trg_func(); -> Option A

4. Quick Check:

   INSTEAD OF triggers need FOR EACH ROW and correct order [OK]

Hint: INSTEAD OF triggers use 'FOR EACH ROW' and 'EXECUTE FUNCTION' [OK]

Common Mistakes:

• Placing INSTEAD OF before ON
• Omitting FOR EACH ROW
• Using BEFORE instead of INSTEAD OF

3. Given the following setup:

CREATE VIEW emp_view AS SELECT id, name FROM employees;
CREATE FUNCTION emp_view_insert() RETURNS trigger AS $$ BEGIN INSERT INTO employees(id, name) VALUES (NEW.id, NEW.name); RETURN NEW; END; $$ LANGUAGE plpgsql;
CREATE TRIGGER trg_emp_insert INSTEAD OF INSERT ON emp_view FOR EACH ROW EXECUTE FUNCTION emp_view_insert();
INSERT INTO emp_view (id, name) VALUES (1, 'Alice');
SELECT * FROM employees WHERE id = 1;

What will the SELECT query return?

medium

A. Empty result because views cannot be inserted

B. No rows

C. Syntax error on INSERT

D. (1, 'Alice')

Solution

1. Step 1: Understand the INSTEAD OF trigger effect

   The trigger function inserts the new row into the base table employees when an insert is done on the view emp_view.

2. Step 2: Trace the INSERT and SELECT

   The INSERT INTO emp_view triggers emp_view_insert(), which inserts (1, 'Alice') into employees. The SELECT then finds this row.

3. Final Answer:

   (1, 'Alice') -> Option D

4. Quick Check:

   INSTEAD OF trigger inserts row into employees [OK]

Hint: INSTEAD OF triggers redirect inserts to base tables [OK]

Common Mistakes:

• Assuming views cannot be inserted into
• Expecting syntax error on insert
• Forgetting trigger function inserts data

4. You wrote this trigger function for an INSTEAD OF UPDATE trigger on a view:

CREATE FUNCTION trg_update() RETURNS trigger AS $$ BEGIN UPDATE employees SET name = NEW.name WHERE id = NEW.id; END; $$ LANGUAGE plpgsql;

When you try to update the view, you get an error. What is the problem?

medium

A. The function does not return a value

B. The trigger should be BEFORE, not INSTEAD OF

C. The UPDATE statement syntax is incorrect

D. The trigger function must be written in SQL, not plpgsql

Solution

1. Step 1: Check trigger function requirements

   Trigger functions must return a value, usually NEW or OLD, depending on the trigger type.

2. Step 2: Identify missing RETURN statement

   The function lacks a RETURN statement, causing an error when the trigger tries to get a result.

3. Final Answer:

   The function does not return a value -> Option A

4. Quick Check:

   Trigger functions must return NEW or OLD [OK]

Hint: Always RETURN NEW or OLD in trigger functions [OK]

Common Mistakes:

• Omitting RETURN in trigger functions
• Confusing INSTEAD OF with BEFORE triggers
• Assuming plpgsql is not allowed

5. You want to create an updatable view for a table products(id, name, price) that only allows updating the price through the view. Which approach using an INSTEAD OF trigger is correct?

hard

A. Create an INSTEAD OF UPDATE trigger that updates all columns in products regardless of input

B. Create a BEFORE UPDATE trigger on the view that blocks changes to name and id

C. Create an INSTEAD OF UPDATE trigger that updates only the price column in products and returns NEW

D. Create a rule instead of a trigger to handle updates on the view

Solution

1. Step 1: Understand the goal

   The view should allow updating only the price column, so the trigger must update only that column in the base table.

2. Step 2: Choose the correct trigger approach

   INSTEAD OF UPDATE triggers on views let you control exactly what updates happen. Updating only price and returning NEW is correct.

3. Final Answer:

   Create an INSTEAD OF UPDATE trigger that updates only the price column in products and returns NEW -> Option C

4. Quick Check:

   INSTEAD OF triggers control updates precisely [OK]

Hint: INSTEAD OF UPDATE triggers can limit which columns update [OK]

Common Mistakes:

• Updating all columns ignoring restrictions
• Using BEFORE triggers on views (not supported)
• Using rules instead of triggers for complex logic